1. Field of the Invention
The present invention relates to a television camera equipped with a continuously-variable-speed electronic shutter function which is used for successively photographing or picking up a moving object or the like.
2. Description of the Related Art
The prior art shutter-equipped television camera belonging to the above category is arranged as shown in FIG. 1, which is disclosed in Japanese Utility Model Laid-open No. Showa 64-29967 (1989). In FIG. 1, an optical lens 11 focuses the image of an object on a solid-state image pick-up device (image sensor) 12. A driving circuit 13 drives the image sensor 12 in a television system, and a shutter driving circuit 14 controls the photo-sensing time of the image sensor 12. A sampling circuit 15 converts the output from the solid-state image sensor 12 into a video signal which is supplied to an average level detecting circuit 16 and an amplifier 17. The average level detecting circuit 16 detects the average level of the video signal and supplies the corresponding signal to the shutter driving circuit 14. The gain of amplifier 17 is controlled by an automatic gain controlling circuit (simply referred to as AGC). Amplifier 17 amplifies the video signal to a predetermined level to be supplied to a succeeding amplifier 19.
The operation of the above prior art will be explained below. In FIG. 1, when the brightness of a subject of the optical lens 11 changes, the output from the solid-state image sensor 12 changes. This leads to a change in the video signal output from the sampling circuit 15. The average level detecting circuit 16 detects this change as an average level and the corresponding control signal to the shutter driving circuit 14 (i.e. the image pick-up system operates in only the AGC mode). When the accumulation time T is lower than upper limit time Tu, both the gain G and the accumulation time T (i.e. the shutter amount) are controlled in accordance with the brightness information Ev to control the photo-sensing time of the solid-state image sensor 12. The amplifier 17 and AGC 18 amplify the video signal output from the sampling circuit 15 so it is maintained at a predetermined level; the amplified signal is supplied to the succeeding amplifier 19.
In this way, the prior art shutter-equipped television camera can automatically adjust the photo-sensing degree or sensitivity of the image sensor 12 in accordance with a change in the brightness of the subject.
However, the prior art shutter-equipped television camera, in which the shutter driving circuit 14 provides control with a reduced number of steps using an up/down counter and an ROM with a low-accuracy shutter number, cannot also enhance the accuracy of the average level detecting circuit 16 and hence cannot responds to an extreme change in the brightness of the subject.